Correction of Misalignment of Antennas in a Logging Tool

ABSTRACT

Misalignment of the transmitter and receiver antennas of an induction logging tool is determined by positioning the logging tool with a conducting loop magnetically coupled to the transmitter antenna and/or the receiver antenna, and activating the transmitter at a plurality of rotational angles. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 11/340,785 filed on 26 Jan. 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to the field of apparatus design in thefield of oil exploration. In particular, the present invention describesa method for calibrating multicomponent logging devices used fordetecting the presence of oil in boreholes penetrating a geologicalformation.

2. Description of the Related Art

Electromagnetic induction resistivity well logging instruments are wellknown in the art. Electromagnetic induction resistivity well logginginstruments are used to determine the electrical conductivity, and itsconverse, resistivity, of earth formations penetrated by a borehole.Formation conductivity has been determined based on results of measuringthe magnetic field of eddy currents that the instrument induces in theformation adjoining the borehole. The electrical conductivity is usedfor, among other reasons, inferring the fluid content of the earthformations.

In general, when using a conventional induction logging tool withtransmitters and receivers (induction coils) oriented only along theborehole axis, the hydrocarbon-bearing zones are difficult to detectwhen they occur in multi-layered or laminated reservoirs. Thesereservoirs usually consist of thin alternating layers of shale and sandand, oftentimes, the layers are so thin that due to the insufficientresolution of the conventional logging tool they cannot be detectedindividually. In this case the average conductivity of the formation isevaluated.

Conventional induction well logging techniques employ antennas wound onan insulating mandrel. One or more transmitter antennas are energized byan alternating current. The oscillating magnetic field produced by thisarrangement results in the induction of currents in the formations whichare nearly proportional to the conductivity of the formations. Thesecurrents, in turn, contribute to the voltage induced in one or morereceiver antenna. By selecting only the voltage component which is inphase with the transmitter current, a signal is obtained that isapproximately proportional to the formation conductivity. Inconventional induction logging apparatus, the basic transmitter antennaand receiver antenna has axes which are aligned with the longitudinalaxis of the well logging device. This arrangement tends to inducesecondary current loops in the formations that are concentric with thevertically oriented transmitting and receiving antennas. The resultantconductivity measurements are indicative of the horizontal conductivity(or resistivity) of the surrounding formations. There are, however,various formations encountered in well logging which have a conductivitythat is anisotropic. Anisotropy results from the manner in whichformation beds were deposited by nature. For example, “uniaxialanisotropy” is characterized by a difference between the horizontalconductivity, in a plane parallel to the bedding plane, and the verticalconductivity, in a direction perpendicular to the bedding plane. Whenthere is no bedding dip, horizontal resistivity can be considered to bein the plane perpendicular to the bore hole, and the verticalresistivity in the direction parallel to the bore hole. Conventionalinduction logging devices, which tend to be sensitive only to thehorizontal conductivity of the formations, do not provide a measure ofvertical conductivity or of anisotropy.

In a transverse induction logging tools the response of transversalantenna arrays is also determined by an average conductivity, however,the relatively lower conductivity of hydrocarbon-bearing sand layersdominates in this estimation. In general, the volume of shale/sand inthe formation can be determined from gamma-ray or nuclear well loggingmeasurements. Then a combination of the conventional induction loggingtool with transmitters and receivers oriented along the well axis andthe transversal induction logging tool can be used for determining theconductivity of individual shale and sand layers.

One, if not the main, difficulties in interpreting the data acquired bya transversal induction logging tool is associated with vulnerability ofits response to borehole conditions. Among these conditions is thepresence of a conductive well fluid as well as wellbore fluid invasioneffects

In the induction logging instruments the acquired data quality dependson the formation electromagnetic parameter distribution (conductivity)in which the tool induction receivers operate. Thus, in the ideal case,the logging tool measures magnetic signals induced by eddy currentsflowing in the formation. Variations in the magnitude and phase of theeddy currents occurring in response to variations in the formationconductivity are reflected as respective variations in the outputvoltage of receivers. In the conventional induction instruments thesereceiver induction antenna voltages are conditioned and then processedusing analog phase sensitive detectors or digitized by digital to analogconverters and then processed with signal processing algorithms. Theprocessing allows for determining both receiver voltage amplitude andphase with respect to the induction transmitter current or magneticfield waveform.

There are a few hardware margins and software limitations that impact aconventional transversal induction logging tool performance and resultin errors appearing in the acquired data.

The general problems encountered are discussed in U.S. Pat. No.6,734,675 and U.S. Pat. No. 6,586,939 to Fanini et al., both having thesame assignee as the present application and the contents of which areincorporated herein by reference. Fanini '939 discloses a transverseinduction logging tool having a transmitter and receiver for downholesampling of formation properties, the tool having a symmetrical shieldedsplit-coil transmitter antenna and a bucking coil interposed between thesplit transmitter coils to reduce coupling of the transmitter timevarying magnetic field into the receiver. The tool provides symmetricalshielding of the coils and grounding at either the transmitter orreceiver end only to reduce coupling of induced currents into thereceived signal. The tool provides an insulator between receiverelectronics and the conductive receiver housing having contact withconductive wellbore fluid, to reduce parasitic current flowing in a loopformed by the upper housing, feed through pipe, lower housing andwellbore fluid adjacent the probe housing or mandrel. An internalverification loop is provided to track changes in transmitter current inthe real and quadrature component of the received data signal.

Fanini '675 discloses a transverse induction logging tool having atransmitter and receiver for downhole sampling of formation properties,the tool having a symmetrical shielded split-coil transmitter antennaand a bucking coil interposed between the split transmitter coils toreduce coupling of the transmitter time varying magnetic field into thereceiver. The tool provides symmetrical shielding of the coils andgrounding at either the transmitter or receiver end only to reducecoupling of induced currents into the received signal. The tool providesan insulator between receiver electronics and the conductive receiverhousing having contact with conductive wellbore fluid, to reduceparasitic current flowing in a loop formed by the upper housing, feedthrough pipe, lower housing and wellbore fluid adjacent the probehousing or mandrel. An internal verification loop is provided to trackchanges in transmitter current in the real and quadrature component ofthe received data signal.

SUMMARY OF THE INVENTION

One embodiment of the invention is an apparatus for evaluating aninduction logging tool for use in a borehole. The logging tool includesa current source which activates a transmitter antenna on the loggingtool and a receiver antenna spaced apart axially from the transmitterantenna wherein the receiver antenna produces a signal in response tothe activation of the transmitter coil. The apparatus includes at leastone conducting loop which magnetically coupled to the transmitterantenna and/or the receiver antenna. The signal produced by the receivercoil is indicative of a misalignment between the transmitter antenna andthe receiver antenna. A single conducting loop may encompass both thetransmitter antenna and the receiver antenna. Two electrically connectedconducting loops may be used with one of the loops encompassing thetransmitter antenna and the other loop encompassing the receiverantenna. The transmitter antenna may have an axis substantially parallelto or substantially orthogonal to a longitudinal axis of the loggingtool. The receiver antenna may have an axis substantially parallel to orsubstantially orthogonal to a longitudinal axis of the logging tool. Theapparatus may further include a support which rotates the logging toolthrough a plurality of angles with the transmitter being activated ateach of the angles, and a processor which estimates from the signals ateach of the rotational angles the angle of misalignment. Themisalignment angle may be estimated based on a rotational angle at whichthe signal is substantially zero.

Another embodiment of the invention is a method of evaluating aninduction logging tool for use in a borehole. A transmitter antenna onthe tool is activated. A signal is provided using a receiver antenna inresponse to the activation of the transmitter antenna. Magnetic couplingis provided between a conducting loop and the transmitter antenna and/orreceiver antenna. From the signal, a misalignment angle between thetransmitter coil and the receiver coil. The method includes using atleast one conducting loop for axially encompassing the transmitterantenna and/or the receiver antenna. The encompassing may be done byusing a single conducting loop. The encompassing may be done by using afirst loop for encompassing the transmitter antenna and a second loopfor encompassing the receiver antenna. The transmitter antenna may havean axis that is substantially parallel to or orthogonal to alongitudinal axis of the tool. The receiver antenna may have an axisthat is substantially parallel to or orthogonal to a longitudinal axisof the tool. The logging tool may be rotated through a plurality ofangles with the transmitter being activated at each of the angles, andthe misalignment angle may be estimated from the signals at each of theangles. The misalignment angle may be estimated from an angle at whichthe signal is substantially zero.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood with reference to theaccompanying figures in which like numerals refer to like elements andin which:

FIG. 1 (prior art) shows schematically a wellbore extending into alaminated earth formation, into which wellbore an induction logging toolas used according to the invention has been lowered;

FIG. 2A (prior art) illustrates a conventional resistivity measurementin the vertical direction;

FIG. 2B (prior art) illustrates a resistivity measurement in thehorizontal direction;

FIG. 3 is an overall flow chart of the procedures of the presentinvention;

FIG. 4 illustrates an assembly for calibrating of transverse arrays in alogging tool;

FIG. 5 illustrates an assembly for calibrating longitudinal arrays in alogging tool;

FIGS. 6-7 illustrate assemblies for calibrating XY cross-componentarrays; and

FIGS. 8-9 illustrate assemblies for calibrating XZ cross-componentarrays.

DETAILED DESCRIPTION OF THE INVENTION

The instrument structure provided by the present invention enablesincreased stability and accuracy in an induction wellbore logging tooland its operational capabilities, which, in turn, results in betterquality and utility of wellbore data acquired during logging. Thefeatures of the present invention are applicable to improve thestructure of a majority of known induction tools.

The invention will now be described in more detail and by way of examplewith reference to the accompanying drawings. FIG. 1 schematically showsa wellbore 1 extending into a laminated earth formation, into whichwellbore an induction logging tool as used according to the presentinvention has been lowered. The wellbore in FIG. 1 extends into an earthformation which includes a hydrocarbon-bearing sand layer 3 locatedbetween an upper shale layer 5 and a higher conductivity than thehydrocarbon bearing sand layer 3. An induction logging tool 9 used inthe practice of the invention has been lowered into the wellbore 1 via awire line 11 extending through a blowout preventor 13 (shownschematically) located at the earth surface 15. The surface equipment 22includes an electric power supply to provide electric power to the setof coils 18 and a signal processor to receive and process electricsignals from the receiver coils 19. Alternatively, the power supplyand/or signal processors are located in the logging tool.

The relative orientation of the wellbore 1 and the logging tool 9 withrespect to the layers 3, 5, 7 is determined by two angles, one of whichθ as shown in the FIG. 1. For determination of these angles see, forexample, U.S. Pat. No. 5,999,883 to Gupta, et al. The logging tool 9 isprovided with a set of transmitter coils 18 and a set of receiver coils19, each set of coils 18, 19 being connected to surface equipment 22 viasuitable conductors (not shown) extending along the wire line 11.

Each set of coils 18 and 19 includes three coils (not shown), which arearranged such that the set has three magnetic dipole moments in mutuallyorthogonal directions, that is, in x, y and z directions. The three-coiltransmitter coil set transmits T_(x), T_(y) and T_(z). The receiver coilreceives R_(x), R_(y) and R_(z) plus the cross components, R_(xy),R_(xz) and R_(zy). Thus, coil set 18 has magnetic dipole moments 26 a,26 b, 26 c, and coil set 19 has magnetic dipole moments 28 a, 28 b, 28c. In one embodiment the transmitter coil set 18 is electricallyisolated from the receiver coil set 19. In an alternative embodiment,each coil in transmitter coil set 18 electrically isolated from eachother and each coil in receiver coil set 19 electrically isolated fromeach other. The coils with magnetic dipole moments 26 a and 28 a aretransverse coils, that is they are oriented so that the magnetic dipolemoments are oriented perpendicular to the wellbore axis, whereby thedirection of magnetic dipole moment 28 a is opposite to the direction ofmagnetic dipole moment 26 a. Furthermore the sets of coils 18 and 19 arepositioned substantially along the longitudinal axis of the logging tool9.

As shown in FIG. 2A, conventional induction logging tools provide asingle transmitter and receiver coil that measure resistivity in thehorizontal direction. In the conventional horizontal mode, as shown inFIG. 2A, the resistivities of adjacent high resistivity sand and lowresistivity shale layers appear in parallel, thus the resistivitymeasurement is dominated by low resistivity shale. As shown in FIGS. 1and 2B, in the present invention a transverse coil is added to measureresistivity in the vertical direction. In the vertical direction, theresistivity of the highly resistive sand and low resistivity shale areappear in series and thus the vertical series resistivity measurement isdominated by the resistivity of the highly resistive sand.

For ease of reference, normal operation of the tool 9, as shown in FIGS.1 and 2B, will be described hereinafter only for the coils having dipolemoments in the x-direction, i.e. dipole moments 26 a and 28 a. Duringnormal operation an alternating current of a frequency f₁ is supplied bythe electric power supply of surface equipment 22 to transmitter coilset 18 so that a magnetic field with magnetic dipole moment 26 a isinduced in the formation. In an alternative embodiment, the frequency isswept through a range f₁ through f₂. This magnetic field extends intothe sand layer 3 and induces a number of local eddy currents in the sandlayer 3. The magnitude of the local eddy currents is dependent upontheir location relative to the transmitter coil set 18, the conductivityof the earth formation at each location, and the frequency at which thetransmitter coil set 18 is operating. In principle the local eddycurrents act as a source inducing new currents, which again inducefurther new currents, and so on. The currents induced into the sandlayer 3 induces a response magnetic field in the formation, which is notin phase with the transmitted magnetic field, but which induces aresponse current in receiver coil set 19. The magnitude of the currentinduced in the sand layer 3 depends on the conductivity of the sandlayer 3, the magnitude of the response current in receiver coil set 19.The magnitude also depends on the conductivity and thereby provides anindication of the conductivity of the sand layer 3. However, themagnetic field generated by transmitter coil set 18 not only extendsinto sand layer 3, but also in the wellbore fluid and in the shalelayers 5 and 7 so that currents in the wellbore fluid and the shalelayers 5 and 7 are induced.

The overall procedures of the present invention used to ensure properfunctioning of a deployed multicomponent induction logging tool issummarized in FIG. 3. Calibration of the instrument's arrays is done,particularly estimating its transfer coefficient 101. Subsequently, afinal verification of the tuning and calibration consistency isperformed 103.

Turning now to FIG. 4, one arrangement of the alignment loop isdiscussed. Shown therein is an alignment loop 501 surrounding an arraycharacterized by the transmitter antenna 504 directed along an Xdirection (T_(x)) and the receiver antenna 508 directed along the Xdirection (R_(x)). Bucking coil B_(x) 506 is also shown. This array isdenoted as XX, using a nomenclature in which the first letter signifiesthe orientation direction of the transmitter antenna and the last lettersignifies the orientation direction of the receiver antenna. Thisnomenclature is generally used herein. The XX and YY arrays in themulti-component tool are ideally aligned at 90° from each other. Whenthis alignment is not met, the response of the cross components (XY, YX)are affected by part of the reading of the related main component. Thealignment measuring method of the present invention is based onanalyzing the output of the cross-component system when the tool isrotated inside of an alignment loop. The alignment loop is magneticallycoupled to the transmitter antenna and the receiver antenna.

The alignment loop 501 is a stationary loop, lying so that thelongitudinal axis of the loop and the longitudinal axis of thewell-logging tool are substantially aligned. Its dimensions are such asto obtain substantial inductive coupling with the transmitter as well aswith the receiver of both XX and YY arrays. An important aspect of thepresent invention is that no portion of either the transmitter or thereceiver coil extends beyond the loop. This is in contrast to prior artdevices in which this condition is not satisfied. When this condition isnot satisfied, the resulting calibration is sensitive to the position ofthe calibration loop relative to the transmitter and receiver antennas,and is hence suspect. The arrangement shown in FIG. 5 uses a singlecalibration loop where this condition is satisfied. The singlecalibration loop axially encompasses the transmitter coil and thereceiver coil. A detailed analysis of the signals is given later in thisdocument. The logging tool is supported within the alignment loop bysuitable support (not shown) that has the capability of rotating thelogging tool about its axis through known angles.

FIG. 5 illustrates a loop alignment assembly usable for aligning ZZarrays in a testing device. Transmitter TZ 601, bucking coil BZ 603 andreceiver RZ 605 are disposed along the feed-through pipe 615 and have acommon longitudinal axis. Alignment loop 610 is substantially coaxialwith receiver RZ 605 and substantially centered on RZ. As with thearrangement of FIG. 5, the receiver coil is axially encompassed by d thecalibration loop 610.

Cross component array calibration is discussed next. FIG. 6 illustratesan embodiment for calibration of an XY array using a calibration box.This functions in the same manner as a calibration loop, and may beconsidered to be one. Transmitter 701 and bucking coil 703 are disposedalong the feed-through pipe oriented to produce a magnetic moment in anX-direction. Receiver 705 is disposed along the same feed-through pipehaving an orientation so as to receive components of a magnetic momentin a is disposed along the same feed-through pipe having an orientationso as to receive components of a magnetic moment in Y-direction. Thealignment loop 710 is disposed at an angle of 45° so as to be orientedhalfway between the X-direction and the Y-direction. To simplify theillustration, the box has been depicted without showing it as extendingbeyond the transmitter and receiver coils.

Those skilled in the art would recognize that the alignment loop shownin FIGS. 4 and 6 would be bulky and possible difficult to manage underfield conditions. An embodiment of the present invention that addresesthis problem is discussed next. FIG. 7 illustrated an alternateembodiment for aligning an XY array. Alignment loop 815 is located atthe TX 801, and alignment loop 810 is positioned at the RXYcross-component receiver 805. Both alignment loops are oriented alongthe same direction as their respective transmitter/receiver. A wire 820electrically couples alignment loop 810 and alignment loop 815. Theindividual loops 810 and 815 are easier to handle than a single largebox, and by use of the electrical connection 820, are functionallyequivalent to box 501 of FIG. 4. In the configuration of FIG. 7, theloop 815 axially encompasses the transmitter coil 801 and the loop 810axially encompasses the receiver coil.

FIG. 8 illustrates an assembly for orienting of the XZ cross-componentarray. Transmitter TX 901 and bucking coil BX 903 are disposed along thefeed-through pipe oriented so as to produce a magnetic moment along anX-direction. The receiver RZ 905 is disposed along the feed-through pipeand oriented so as to be receptive to Z-components of magnetic moments.The alignment loop 920 can be positioned centrally between mainX-transmitter 901 and Z-cross-component receiver 905 and tilted 45° withrespect to the tool longitudinal axis 910. The assembly of FIG. 8displays small signals during XZ array calibration. This signal tends todisplay a high sensitivity to the angle.

FIG. 9 illustrates an alternate embodiment for aligning the XZcross-component array. As in the apparatus shown in FIG. 7, two loopsare used. Transmitter TX 1001 and bucking coil BX 1003 are disposedalong the feed-through pipe oriented so as to produce a magnetic momentalong an X-direction. The receiver RZ 1005 is disposed along thefeed-through pipe and oriented so as to be receptive to Z-components ofmagnetic moments. Alignment loop 1010 is centered on transmitter TX1001, and alignment loop 1015 is coaxial with receiver RZ 1005. A wire1020 electrically couples alignment loop 1010 and alignment loop 1015.In contrast to the assembly of FIG. 8, calibration using two alignmentdevices displays a large signal for the XZ array calibration.

We next discuss in detail the use of the alignment loop for establishingthe coil orientation. When examining a cross-component array, the XY orYX response obtained by rotating the tool inside of the alignment loophas a zero-crossing each time that either a transmitter or a receivercoil is perpendicular to the plane of the loop. Whichever coil(transmitter or receiver) is substantially aligned with the loop(enclosed in the same plane) experiences a maximum coupling with thealignment loop. When the position of the aligned coil is varied aroundthe point of alignment with the alignment loop, the coupling responsebetween them undergoes a slow change corresponding to the variation. Thenon-aligned coil experiences a minimum coupling with the alignment loop.When the position of the non-aligned coil is varied around this point ofminimal coupling, the coupling experiences an abrupt change. Thecoupling becomes zero when the non-aligned coil achievesperpendicularity with the alignment loop. A practitioner in the artwould recognize that the zero-crossings of the coupling response aresignificantly affected by the coil that is at right angle to thealignment loop, regardless of whether the perpendicular coil is areceiver or a transmitter. The substantially aligned coil plays littleor no role in the production of a zero-crossing. The angle betweensuccessive zero crossings thereby represents an alignment angle betweenthe two related coils.

Mathematically, the inductive coupling between two coils resembles acosine function of the angle between them. Thus, the coupling responsesystem of coils made by an aligned system of cross components and analignment loop is given by the following expression: $\begin{matrix}{{{R(\phi)} = {K \cdot {\cos(\phi)}}}{\cdot {{\cos\left( {\phi - \frac{\pi}{2}} \right)}.}}} & (1)\end{matrix}$Applying trigonometric identities, Eq. (1) can be simplified to$\begin{matrix}{{{R(\phi)} = {K \cdot {\cos(\phi)} \cdot {\sin(\phi)}}},{{and}\quad{since}}} & (2) \\{{{{\sin(\phi)} \cdot {\cos(\phi)}} = {\frac{1}{2}{\sin\left( {2 \cdot \phi} \right)}}},{{it}\quad{follows}\quad{that}}} & (3) \\{{R(\phi)} = {K \cdot \frac{1}{2} \cdot {{\sin\left( {2 \cdot \phi} \right)}.}}} & (4)\end{matrix}$Eqn. (4) illustrates that there are two cycles of variation for eachcycle of tool rotation.

By considering a misalignment angle β between transmitter and receiver,the response function can now be expressed as $\begin{matrix}{{{R\left( {\phi,\beta} \right)} = {K \cdot {\cos(\phi)} \cdot {\cos\left( {\phi - \frac{\pi}{2} + \beta} \right)}}},} & (5)\end{matrix}$where each cosine function characterizes the response of the individualcross component coils. It is easy to see that $\begin{matrix}{{{{R\left( {\phi,\beta} \right)} = 0},{when}}{\phi = {n \cdot \frac{\pi}{2}}}{{or}\quad{when}}} & (6) \\{{{\phi - \frac{\pi}{2} + \beta} = {{{n \cdot \frac{\pi}{2}}\quad{with}\quad n} = {\pm 1}}},2,3,\ldots} & {{Eq}.\quad(7)}\end{matrix}$According to eqn. (7), the angle between successive zero-crossingsrepresents the alignment angle among the cross component coils. Anintuitive graphical approach can therefore be used to measure themisalignment angle between transmitter and receiver.

Alternatively, the misalignment angle can be obtained simply by using atrigonometric regression function to analyze the response of the system.Applying trigonometric identities to Eqn. (5), the response of themisaligned system can be written as $\begin{matrix}{{{R\left( {\phi,\beta} \right)} = {{K \cdot {\cos(\phi)} \cdot {\sin(\phi)} \cdot {\cos(\beta)}} + {K \cdot {\cos^{2}(\phi)} \cdot {\sin(\beta)}}}}{{R\left( {\phi,\beta} \right)} = {{K \cdot \frac{1}{2} \cdot {\sin\left( {2 \cdot \phi} \right)} \cdot {\cos(\beta)}} + {K \cdot {\cos^{2}(\phi)} \cdot {\sin(\beta)}}}}{{R\left( {\phi,\beta} \right)} = {{\frac{K}{2} \cdot {\sin\left( {{2 \cdot \phi} + \beta} \right)}} + {\frac{K}{2} \cdot {\sin(\beta)}}}}} & (8)\end{matrix}$The last expression in eqn. (8) indicates that a graphicalrepresentation of the coupling response of the misaligned crosscomponent system resembles a sinusoidal function. The period of thissinusoid equals 180° and has offsets on both the abscissa and theordinate. The offset on the abscissa is β, and the offset on theordinate is (K/2)sin(β). Also, the coupling response is of the form Asin(x+B)+C, where A=K/2, B=β and C=(K/2)(sin(β). The coefficient Bobtained with such fitting represents the misalignment angle. The crosscomponent response can thus be fit to this curve.

Implicit in the control and processing of the data is the use of acomputer program on a suitable machine readable medium that enables theprocessor to perform the control and processing. The machine readablemedium may include ROMs, EPROMs, EEPROMs, Flash Memories and Opticaldisks.

While the foregoing disclosure is directed to the preferred embodimentsof the invention, various modifications will be apparent to thoseskilled in the art. It is intended that all variations within the scopeand spirit of the appended claims be embraced by the foregoingdisclosure.

The following definitions are helpful in understanding the scope of theinvention:

-   alignment: the proper positioning or state of adjustment of parts in    relation to each other;-   calibrate: to standardize by determining the deviation from a    standard so as to ascertain the proper correction factors;-   coil: one or more turns, possibly circular or cylindrical of a    current-carrying conductor capable of producing, a magnetic field;-   EAROM: electrically alterable ROM;-   Encompass: to enclose completely-   EPROM: erasable programmable ROM;-   flash memory: a nonvolatile memory that is rewritable;-   machine readable medium: something on which information may be    stored in a form that can be understood by a computer or a    processor;-   misalignment: the condition of being out of line or improperly    adjusted;-   Optical disk: a disc shaped medium in which optical methods are used    for storing and retrieving information;-   Position: an act of placing or arranging; the point or area occupied    by a physical object-   Quadrature: 90° out of phase; and-   ROM: Read-only memory.

1. An apparatus for evaluating an induction logging tool, the loggingtool comprising: (a) a source configured to activate a transmitterantenna on the induction logging tool; and (b) a receiver antennaconfigured to produce a signal in response to the activation of thetransmitter antenna; the apparatus comprising: (c) at least oneconducting loop magnetically coupled to at least one of (A) thetransmitter antenna, and (B) the receiver antenna; wherein the signal isindicative of a misalignment angle between the transmitter antenna andthe receiver antenna.
 2. The apparatus of claim 1 wherein the receiverantenna is spaced actually apart from the transmitter antenna.
 3. Theapparatus of claim 1 wherein the at least one conducting loop comprisesa single conducting loop axially encompassing the transmitter antennaand the receiver antenna.
 4. The apparatus of claim 1 wherein the atleast one conducting loop comprises a first conducting loop axiallyencompassing the transmitter antenna and a second conducting loopelectrically connected to the first loop, the second conducting loopaxially encompassing the receiver antenna.
 5. The apparatus of claim 1wherein the transmitter antenna has an axis that is one of (i)substantially parallel to a longitudinal axis of the logging tool, and(ii) substantially orthogonal to a longitudinal axis of the loggingtool.
 6. The apparatus of claim 1 wherein the receiver antenna has anaxis that is one of (i) parallel to a longitudinal axis of the loggingtool, and (ii) substantially orthogonal to a longitudinal axis of thelogging tool.
 7. The apparatus of claim 1 further comprising: (i) asupport configured to rotate the logging tool through a plurality ofangles, the transmitter being configured to be activated at each of theplurality of rotational angles, and (ii) a processor configured toestimate from the signals at each of the plurality of rotational anglesthe misalignment angle.
 8. The apparatus of claim 6 wherein theprocessor is configured to determine the misalignment angle at least inpart by using a relation of the form:${R\left( {\phi,\beta} \right)} = {K \cdot {\cos(\phi)} \cdot {\cos\left( {\phi - \frac{\pi}{2} + \beta} \right)}}$where K is a gain factor, φ is an angle between one of the antennas anda plane defined by the at least one conducting loop, and β is themisalignment angle.
 9. The apparatus of claim 6 wherein the processor isconfigured to determine the misalignment angle based on estimating arotational angle at which the signal is substantially zero.
 10. A methodof evaluating an induction logging tool, the method comprising: (a)activating a transmitter antenna on the induction logging tool; (b)magnetically coupling the transmitter antenna and a receiver antennawith at least one conducting loop; (c) using the receiver antenna toprovide a signal in response to the activation of the transmitterantenna; (d) estimating from the signal a misalignment angle between thetransmitter antenna and the receiver antenna
 11. The method of claim 10wherein the positioning of the logging tool further comprises using asingle conducting loop for axially encompassing the transmitter antennaand the receiver antenna.
 12. The method of claim 10 wherein thepositioning of the logging tool further comprises: (i) using a firstconducting loop for axially encompassing the transmitter antenna; and(ii) using a second conducting loop electrically connected to the firstloop for axially encompassing the receiver antenna.
 13. The method ofclaim 10 wherein the transmitter antenna has an axis that is one of (i)substantially parallel to a longitudinal axis of the logging tool, and(ii) substantially orthogonal to a longitudinal axis of the loggingtool.
 14. The method of claim 10 wherein the receiver antenna has anaxis that is one of (i) substantially parallel to a longitudinal axis ofthe logging tool, and (ii) substantially orthogonal to a longitudinalaxis of the logging tool.
 15. The method of claim 10 further comprising:rotating the logging tool through a plurality of angles, the transmitterantenna being activated at each of the plurality of rotational angles,wherein estimating the misalignment angle further comprises usingsignals at each of the plurality of rotational angles.
 16. The method ofclaim 15 wherein determining the misalignment further comprises using arelation of the form:${R\left( {\phi,\beta} \right)} = {K \cdot {\cos(\phi)} \cdot {\cos\left( {\phi - \frac{\pi}{2} + \beta} \right)}}$where K is a gain factor, φ is an angle between one of the antenna and aplane defined by the at least one conducting loop, and β is themisalignment angle.
 17. The method of claim 15 wherein estimating themisalignment angle further comprises estimating a rotational angle atwhich the signal is substantially zero.